Field of the Invention
This invention concerns a method for producing chemicals of high purity. More particularly, it relates to a method and apparatus for producing chemicals of high purity by separating and removing non-volatile impurities contained in trace amounts in chemicals.
Recent developments in semiconductors, optical fibers, fine ceramics, etc., have created a need for higher purity industrial materials, particularly those low in non-volatile impurities such as metal impurities and high molecular weight impurities. Further, recent developments in microanalysis have made it possible to analyze metals to parts per billion or parts per trillion. High purity for the various analytic reagents used in microanalysis is required.
Distillation has usually been employed to purify starting materials and analytic reagents which are liquid and thermally stable. For instance, Japanese Patent Laid-Open Publication No. Sho 61-191502 describes a method of purifying hydrofluoric acid containing arsenic compounds by adding a halogen flouride to the hydrofluoric acid to react with the arsenic compound and then purifying the hydrofluoric acid by distillation.
Purification by distillation includes a procedure for heating a liquid to its boiling point, and cooling the evolved vapors back to the liquid state. For high purification, redistillation can be effected in one distillation step by enlarging the contact section of the vapors and the liquid between the heating portion and the condensing portion. If the leaching of impurities from the distillation device is eliminated and if there is no entrainment in which minute droplets are scattered in the vapors and carried in the gas stream, it would be possible to produce chemicals of super high purity, for example, with regard to metal impurities in the range of parts per billion or parts per trillion.
However, the phenomenon of entrainment in purification by distillation cannot be prevented completely even if a rectification means is employed. If it is intended to reduce the phenomenon of entrainment, the distillation or rectification has to be carried out very slowly, whereby the yield per unit time is decreased. Thus, it is extremely uneconomical from an industrial point of view to purify chemicals to super high purity by distillation or rectification. At present, to attain a proper degree of purity, small amounts of the chemicals in analytical reagents for which super high purity is required are purified in the laboratory each time they are needed.